Pilot-scale generation of plasma processed air and its influence on microbial count, microbial diversity, and selected quality parameters of dried herbs

Herbal teas can cause diseases, when their naturally high microbial load is not inactivated via proper brewing. Cold plasma as effective and gentle decontamination technique was investigated for dried peppermint, stinging nettle, and lemongrass. Microwave-driven plasma processed air (up to 7.5 min) significantly reduced total viable counts up to 1.6 log10 CFU/g (initial counts were 8.02, 7.43, and 7.44 log10 CFU/g for peppermint, stinging nettle, and lemongrass, respectively). Spore-forming bacteria, Enterobacteriaceae, yeasts, and molds were inactivated up to 2 log10 CFU/g. MALDI-ToF MS analyses showed a 30-100% reduction in microbial diversity as result of the plasma treatment. Color of most samples was visibly reduced in greenness and antioxidants, phenols, flavonoids, and free amino groups mostly decreased by plasma treatment. Overall, use of plasma processed air could be a suitable tool to decontaminate dried herbs at larger scale but has to be improved by testing different sources, gases, or times. Industrial relevance: Cold atmospheric plasma can inactivate herb-associated microorganisms. Its possible effect on quality parameters such as flavor, color or phenolic content should be further studied and reduced, as particularly gentle decontamination processes that preserve the natural appearance of the food are accepted by the consumer. It seemed, that the intensity of the treatment can be reduced by pulsed plasma applications. There is great potential for application on an industrial scale. Peppermint seems to be the most suitable in this context, as it showed the highest microbial reduction together with low quality changes and is widely used, including in so-called cold brew teas, which require particular safety.